[Paper Review] Exciton, trion and localized exciton in monolayer Tungsten Disulfide

A. Hichri, Imen Ben Amara+2 more|arXiv (Cornell University)|Sep 19, 2016

2D Materials and Applications1 references7 citations

TL;DR

This study investigates neutral excitons, charged trions, and localized excitons in monolayer tungsten disulfide (WS2) using an effective mass model parameterized by ab-initio calculations. It demonstrates that strong electron-hole correlations and dielectric screening lead to trion formation and that free excitons are significantly suppressed due to trapping in disordered potential wells, forming localized excitons, with emission tunable via temperature, excitation, and electron injection.

ABSTRACT

The ultrathin transition metal dichalcogenides (TMDs) have emerged as promising materials for various applications using two dimensional (2D) semiconductors. They have attracted increasing attention due to their unique optical properties originate from neutral and charged excitons. Here, we report negatively charged exciton formation in monolayer TMDs, notably tungsten disulfide WS2. Our theory is based on an effective mass model of neutral and charged excitons, parameterized by ab-initio calculations. Taking into the account the strong correlation between the monolayer WS2 and the surrounding dielectric environment, our theoretical results are in good agreement with one-photon photoluminescence (PL) and reflectivity measurements. We also show that the exciton state with p-symmetry, experimentally observed by two-photon PL emission, is energetically below the 2s-state. We use the equilibrium mass action law, to quantify the relative weight of exciton and trion PL. We show that exciton and trion emission can be tuned and controlled by external parameters like temperature, pumping and injection electrons. Finally, in comparison with experimental measurements, we show that exciton emission in monolayer tungsten dichalcogenides is substantially reduced. This feature suggests that free exciton can be trapped in disordered potential wells to form a localized exciton and therefore offers a route toward novel optical properties.

Motivation & Objective

• To understand the formation and optical properties of neutral excitons and charged trions in monolayer WS2.
• To explain the suppression of free exciton emission observed in experiments.
• To investigate the role of electron-hole correlations and dielectric screening in determining excitonic states.
• To quantify the relative contributions of exciton and trion photoluminescence using mass action law.
• To explore the potential for tuning excitonic emission through external parameters like temperature and carrier injection.

Proposed method

• An effective mass model is developed for neutral and charged excitons in monolayer WS2.
• Model parameters are extracted from ab-initio calculations to account for electron-hole interactions and dielectric screening.
• The model incorporates the influence of the surrounding dielectric environment on excitonic states.
• One-photon photoluminescence and reflectivity measurements are used to validate theoretical predictions.
• Two-photon photoluminescence data are analyzed to confirm the energy ordering of p-symmetry and 2s excitonic states.
• The equilibrium mass action law is applied to calculate the relative weights of exciton and trion emission under varying conditions.

Experimental results

Research questions

• RQ1 Why is free exciton emission substantially reduced in monolayer WS2 despite strong excitonic effects?
• RQ2 How do electron-hole correlations and dielectric screening influence the formation of trions in WS2?
• RQ3 What is the energy ordering between the p-symmetry and 2s excitonic states in monolayer WS2?
• RQ4 To what extent can exciton and trion emission be tuned by external parameters such as temperature and carrier injection?
• RQ5 What is the role of disorder in transforming free excitons into localized excitons?

Key findings

• The theoretical model shows excellent agreement with one-photon photoluminescence and reflectivity measurements in monolayer WS2.
• The p-symmetry excitonic state is found to be energetically below the 2s state, consistent with two-photon PL observations.
• Trion formation is strongly influenced by electron-hole correlations and the dielectric environment, leading to enhanced charged exciton stability.
• Free exciton emission is significantly reduced due to trapping in disordered potential wells, forming localized excitons.
• The relative intensity of exciton and trion photoluminescence can be quantitatively tuned via temperature, excitation power, and electron injection.
• The results suggest that localized excitons in WS2 offer a pathway to engineer novel optical properties in 2D semiconductors.